Epoxy resins are among the most important industrial polymers in the world and are used in large quantities in the production of adhesives, paints and coatings, and matrix resins. The core substrate in the production of epoxy resins is typically 2,2-bis(4-hydroxyphenyl)isopropylidiene (bisphenol A). The main monomer used in the epoxy resin industry is the diglycidyl ether of bisphenol A, (2,2-Bis(4-glycidyloxyphenyl)propane), which represents more than 75% of the resin used in industrial applications. This monomer is usually prepared from 2,2-Bis(4-hydroxyphenyl)isopropylidiene (bisphenol A) and epichlorohydrin using a strong base such as sodium hydroxide. Alternative synthetic methods have been developed such as allylating bisphenol A followed by epoxidization.
One significant application for epoxy resins derived from the diglycidyl ether of bisphenol A is in glass fiber reinforced laminates as rigid or flexible circuit board substrates used in a variety of industrial and consumer electronic products and electronic components. These materials must be fire resistant to meet safety requirements. The approach to render these materials fire resistant has been to use a variety of additives such as brominated compounds, phosphorous containing compounds, aluminum derivatives, melamine cyanurate, metal phosphinates and combinations thereof. Due to environmental considerations, some of the more commonly used halogenated flame retardants are being banned from use because they can leach out into the environment and are toxic. As is the case with most additives for polymers, the other flame retardants suffer from the same problem, it is just that they have not received the attention that the halogenated systems have, but many of them are toxic and they all are subject to leach out of the host resin. Thus there is a need to render epoxy derived resins fire resistant in a way that is practical, cost effective and environmentally friendly.
Processes for preparing flame retardant epoxy resins disclosed thus far are focused on the reaction of an epoxy resin with a flame-retardant additive or a curing agent or chain extender. An approach that produces an inherently flame retardant epoxy based monomer, oligomer, polymer, or co-polymer would be ideal. Phosphonate polymers, copolymers, oligomers and co-oligomers having a wide variety of chemical structures which may contain reactive end groups such as hydroxyl groups are known. However, prior art, which has disclosed the reaction of bisphenol A and epichlorohydrin with sodium hydroxide base is not applicable to phosphonate monomers or polymers, copolymers, oligomers or co-oligomers because the use of a strong base to conduct the synthesis causes hydrolysis of the phosphonate groups leading to chain cleavage (and thus reduction of molecular weight) and phosphonic acid groups as well as other unwanted reactions leading to a complex mixture of by-products.